1. Field of the Invention
This invention relates, generally, to the treatment of wastewater, and more particularly to creating a local current in a small wastewater treatment system, such as is suitable for residential use, and to help reduce the build up of solids at certain locations within the wastewater treatment system.
2. Prior Art
Numerous devices are known in the art for the treatment of wastewater. Most treatment systems have a configuration which includes a first chamber for aeration of the wastewater, a second chamber for removing suspended solids, an inlet for feeding the wastewater into the tank, and an outlet for letting the treated water out. The first chamber is a treatment chamber where much of the biological activity in a wastewater treatment system takes place. In the treatment chamber, air is required for microbes to aerobically operate on the wastes in the wastewater (both solid and liquid wastes). Air is usually injected into the treatment system in a variety of locations in the treatment tank. The release locations are chosen to insure that sufficient air (oxygen) is present and fairly evenly distributed throughout the entire treatment chamber to support aerobic activity. Another concern is that adequate mixing takes place in the treatment system to keep most of the solids present in the wastewater suspended to assist microbial degradation. Consequently, the treatment area tends to be a fairly high energy environment.
A wastewater treatment system generally has a second tank, a clarifier chamber. Water is removed from the treatment system through the clarifier chamber. However, it is desired that the removed, or treated water, be relatively free of solids. Consequently, the clarifier chamber is designed as a low energy environment, or quiescent environment, to allow solids present in the water in the clarifier chamber to settle out.
One common type of small (generally less than 2500 gallon capacity) wastewater treatment systems incorporates both the clarifier chamber and treatment chamber into a single tank. One type of small treatment system uses a rectangular tank having a partition therein to create two chambers. The partition does not extend to the floor, thereby allowing the two chambers to be in fluid communication. Another popular treatment system in use employs a single cylindrical tank (or square tank) having an inverted cone clarifier positioned within the tank with radially spaced air pipes delivering air into the tank outside the clarifier, as illustrated in U.S. Pat. No. 4,650,577 to Bradley L. Hansel, herein incorporated by reference (hereafter referred to as the inverted cone system). The area of the tank within the clarifier is a quiescent zone allowing solids to settle out of the water prior to discharge. The area of the tank external to the clarifier is the aeration zone where mixing and oxygen transfer occurs and much of the biological activity takes place. Air is injected into the system through a series of drop lines, which may have fine bubble diffusers positioned on the ends thereof, as disclosed in U.S. Pat. No. 5,266,239 to Drewery, herein incorporated by reference.
In an aerobic treatment system, air must be provided into the system. Some treatment systems include pipes for injecting air into the wastewater in the tank to assist in the aerobic treatment of the fluid, with or without diffusers. One such example is shown in U.S. Pat. No. 4,975,197, herein incorporated by reference. Some systems have a vertical clarifying areas within the tank, such as that shown in U.S. Pat. No. 4,664,795 to Stegall, herein incorporated by reference.
Air injected into the system will create currents within the treatment tank. With multiple release locations, local currents are created about each release site. Some local currents may combine to provide an overall net current within the tank, or may form local eddies, where water circulates within a generally closed volume of the treatment tank. Prior art inverted cone treatment chambers generally had a series of air release sites placed near the bottom of the treatment tank close to the treatment tank wall. These release sites were equally spaced along the outer wall, resulting in a dead zone in the middle of the tank underneath the clarifier opening.
Solids suspended in the wastewater will be aerobically biologically degraded. If the treatment tank contains an area of low fluid velocity (circulatory "dead zones") near the tank bottom, suspended solids will settle out into a sludge onto the tank floor in these zones (zones with insufficient fluid velocity to keep solids in suspension). Circulatory dead zones tend to be oxygen deficient, promoting anoxic degradation of the sludge. Anoxic degradation results in the generation of nitrogen gas generated by the breakdown of nitrates ("denitrification"). Nitrogen gas can build up in the sludge until the sludge, or a portion thereof, is lifted up off the floor back into the treatment tank ("bulking"). Anoxic treatment zones within a treatment system are desirable, as these zones assist in promoting more complete biological degradation.
Circulatory dead zones are not desirable in all locations on the tank bottom. One area where a dead zone is undesirable is directly underneath the clarifier opening. If bulking occurs in this location, the rising solids can result in an undesirable discharge through the clarifier and disturb the clarifier's finction; additionally, a scum can form on the surface of the clarifier and because of the clarifier's quiescent environment, such a scum layer is difficult to remove. Unfortunately, solids tend to settle at this location. The clarifier is designed to allow solids to settle out through the clarifier opening. Consequently, some means must be provided to keep these settling solids from being deposited underneath the clarifier opening.
The prior art attempted to prevent the build up of solids beneath the clarifier opening by providing a diverter (usually a cone or pyramidal shaped deflector) underneath the opening to divert settling solids out of the dead zone and back into the high energy treatment zone. In rectangular tanks, the clarifier is separated from the treatment tank by a common partition (not inclined), with an opening at the bottom of the partition to allow fluid communication. Positioned near this opening in the bottom of the clarifier, was an inclined plane to divert settling solids back into the treatment chamber. These devices were needed as the area beneath the opening of the clarifier tended to be a circulatory dead zone. One such device can be seen in U.S. Pat. No. 5,714,061.
One attempt to prevent a solids build up beneath the clarifier without the use of a diverter is shown in U.S. Pat. No. 5,785,854 to McKinney, herein incorporated by reference. McKinney demonstrates release of air near the sidewall of the treatment tank at a single location to create a particular current within the tank. The current pattern, as shown by McKinney, sweeps across the tank bottom directed toward the air release site thereby sweeping solids, which settle out from the clarifier onto the tank bottom, back into the treatment area. Unfortunately, to create this sweeping pattern, the McKinney current must travel up the sidewall, around the clarifier partition, then down the opposite tank sidewall before sweeping across the tank bottom near the clarifier opening. Such a long current path implies that much of the energy and velocity of the current near the generating source (the air release site) will be lost to overall circulation, making the current sweeping the bottom weak and potentially insufficient to keep solids suspended. Additionally, the McKinney current is generated through air release at a single location in the tank. Air release at a single location can result in insufficient mixing within the entire treatment chamber and inefficient oxygen transfer into the tank as a whole. To address this problem, McKinney suggests using multiple air drops at the same release location. Finally, with use of a single air release site near the sidewall, McKinney indicates that no circulatory dead zones are formed, even though such dead zone, properly placed, are useful.